Railgun accelerators have met with limited success in accelerating projectiles of from 1 gram to about 1 kilogram to velocities of about 7 km/s. Referring to FIG. 1a, a railgun accelerator having a pair of parallel spaced apart conducting rails 1, accelerates a projectile 2 located between the rails 1, by establishing a high current plasma arc or armature 3 between the rails 1, behind the projectile 2.
Under ideal conditions, there is only one current conduction path from rail to rail and it is located immediately behind the projectile 2. The magnetic fields from the currents in the rails 1 couple with the current in the armature and results in a Lorentz force on the plasma, which then results in a hydrodynamic acceleration pressure on the projectile 2.
In reality, arc growth and separation are aggravated by barrel-wall ablation 4 as illustrated in FIG. 1b. Referring also to FIG. 1c, while the projectile 2 continues to be accelerated as it and the plasma arc 3 move down the rails 1, gradual erosion of the launcher causes a secondary arc, or restrike 5, to form in the debris left behind by the first armature 3.
The secondary arc 5 may form right behind the neutral ablation products 4 of the first armature 3 or it may form farther towards the breech of the launcher where the rail-to-rail voltage is higher and the pressure is lower. In either situation, the secondary arc 5 is undesirable because it reduces the propulsive capability of the railgun, thereby limiting the railgun operating velocity.
Specifically, the secondary arc 5 shunts current away from the primary, propulsive, plasmic arc 3 employed to propel the projectile 2. The projectile acceleration force, F, diminishes with the current, I, squared: F=L'I.sup.2 /2, where L' is the inductance gradient of the rail pair. Hence, the propulsive force rapidly decreases as the shunt current grows.
Efforts have been made to accelerate projectiles at velocities greater than 8 to 9 km/s. However, as the velocities increase, the problem of restrike becomes more prevalent and high velocities are difficult to obtain.
In Railgun Development for EOS Applications: A Status Report, by R.S. Hawke, the article discloses that restrike can be reduced by filling the railgun bore with pure hydrogen after the projectile passes. The article, however, does not disclose the ways in which the hydrogen is introduced into the bore, the use of other gases besides hydrogen, or that the projectile itself may be designed to reduce restrike such that gas need not be injected into the railgun.